The present invention relates to a gate turn-off thyristor (GTO).
A GTO is a switching device in which when a positive potential is applied to a gate electrode while a positive voltage is applied between the anode and the cathode, the anode-cathode path is made conductive due to the flow of gate current, and when a negative potential is applied to the gate while an anode current is flowing, part of the anode current flows into the gate so that the anode-cathode path becomes blocked. In this way, the GTO can be easily switched and can be used at a higher voltage and a larger current than power transistors. Therefore, the GTO has recently been getting attention as a high-frequency device capable of handling large power.
An example of conventional GTOs is disclosed in Japanese Laid-Open patent application No. 169973/83 laid open on Oct. 6, 1983; its problems will be described with reference to FIGS. 1 and 2.
As illustrated in FIGS. 1A and 1B, this GTO has a P.sup.+ P.sup.- emitter structure and a multi-emitter structure. Namely, as shown in a plan view of FIG. 1A, a second emitter layer in contact with cathode electrodes is divided into a plurality of substantially rectangular shaped emitter strips. As shown in FIG. 1B, a first emitter layer in contact with an anode electrode is divided into regions (P.sup.+ and P.sup.- regions) having different impurity concentrations.
More specifically, a first N-type base layer 12 is formed on a first P-type emitter layer 11. A second P-type base layer 13 is formed on base layer 12. Further, a plurality of emitter strips 14.sub.1 to 14.sub.n is radially formed on the base layer 13. An anode electrode 15 is formed on the lower surface of the first emitter layer 11. A gate electrode 17 is formed on the upper surface of a second base layer 13, excluding portions thereof where PN-junctions are formed with emitter strips 14.sub.1 to 14.sub.n. Cathode electrodes 16.sub.1 to 16.sub.n are formed on the top surface of the emitter strips 14.sub.1 to 14.sub.n. In order to increase a peak turn-off current I.sub.TGQM while suppressing the local current concentration at the time of the turn-off operation and to allow it to be performed while suppressing the carrier injection from the first P-type emitter layer 11, the emitter layer 11 is formed of regions (P.sup.+ regions) 18 each having a relatively high impurity concentration and regions (P.sup.- regions) 19 each having a relatively low impurity concentration. The region 19 is almost rectangular, similar to emitter strip 14, and is formed immediately below the corresponding emitter strip 14 within a region where the emitter strip is projected.
FIG. 2 is a waveform diagram of a voltage, current and power dissipation in the turn-off operation. When a gate switch is turned on at time t.sub.0 when the GTO is conducting, the gate current i.sub.g increases in the negative direction. Then, the anode current I.sub.A starts decreasing at time t.sub.1. At the same time, the anode-cathode voltage V.sub.A starts increasing. The PN-junction between gate and cathode is recovered at time t.sub.2 and a gate-cathode voltage v.sub.g becomes a maximum negative value. The gate current i.sub.g also becomes its maximum almost simultaneously, thereafter rapidly decreasing. After time t.sub.2, the anode current I.sub.A consists only of a residual charge component. The anode current I.sub.A for this interval is generally called a tailing current.
As the GTO turns off, current is concentrated in a region below the central portion of cathode electrode 16 that is located farthest from the gate electrode due to a lateral resistance effect in P base 13. To prevent this current concentration, a large-power GTO generally has the multi-emitter structure with emitter strips 14.sub.1 to 14.sub.n in which the lateral resistance is small. However, it is very difficult to make characteristics of GTO elements or unit GTOs (GTO region associated with a single emitter strip) uniform. Thus, the current is eventually concentrated to one GTO element at the final stage of the turn-off process. Consequently, the GTO often breaks down due to heat generation. The anode current immediately before breakdown is the peak turn-off current I.sub.TGOM.
As shown in FIG. 1A, in a case where the low impurity concentration region 19 is formed in a region of the first emitter layer 11 just below the emitter strip 14.sub.n, the current concentration in the central portion of each cathode electrode is suppressed and the current is distributed to a peripheral portion near the gate electrode 17 to which a negative bias is applied. Thus, the heat generation decreases and the anode current is preferably taken from gate electrode 17, causing the I.sub.TGQM to be increased. In the P.sup.+ P.sup.- emitter GTO, to further increase the I.sub.TGQM, each low impurity concentration region 19 is formed so that its width X.sub.PE.spsb.- becomes as wide as possible
Since the carrier injection from P emitter 11 is suppressed due to a wider X.sub.PE.spsb.-, the tailing current, which largely contributes to the switching loss, can be reduced, thereby enabling the permissible operating frequency to be increased.
As described above, in the case of a conventional P.sup.+ P.sup.- emitter GTO, by making the X.sub.PE.spsb.- wide, the I.sub.TGQM can be increased and the tailing current can be reduced. In this case, however, a current path between the P emitter 11 and N emitter 14 is likely to spread parallel to the major surface, which decreases the advantage of the formation of the low impurity concentration region 19. Consequently, the tailing current cannot be made sufficiently low unless the X.sub.PE.spsb.- is so widened that the latching current and ON-state voltage increase.